1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus, and more particularly, to a sheet processing apparatus that performs processing on sheets.
2. Description of the Related Art
A conventional image forming apparatus such as a printer may include a sheet processing apparatus that aligns the end portions of sheets on which images have been formed (printed) and performs post-processing such as stapling on the aligned sheets before discharge. Japanese Unexamined Patent Publication No. 2003-073012 discloses a structure in which a sheet processing apparatus is connected to an image forming apparatus and is set above the image forming apparatus.
As an example of such a sheet processing apparatus, an apparatus to be provided on the downstream side of the sheet discharge outlet of the main body of an image forming apparatus is known. This sheet processing apparatus aligns the end portions of sheets that are supplied one by one through a discharge outlet after printing is performed in the main body of the apparatus, and guides the sheets into the opening in a stapling device. The sheet processing apparatus then performs post-processing on the sheets, and discharges the sheets.
Also, to perform precise alignment, a stapling device is normally equipped with a sheet holding portion that prevents a subsequent sheet from pushing an aligned precedent sheet and disturbing the alignment when sheets are being supplied one by one.
In a sheet processing apparatus with such a stapling function, it is essential to perform post-processing without disturbing the alignment of sheets, and discharge the sheets in a bundle without damage.
However, as shown in FIG. 14, a stapling portion 1401 and an intermediate stacking portion 1402 that aligns sheets are normally formed as separate portions. Therefore, there is always a space or a step formed between those two portions.
Also in the stapling portion 1401, as shown in FIG. 15, a space or a step always exists between a stapling portion main body 1401a and a staple cartridge portion 1402b. 
Therefore, when the end portion of a sheet with a large curl enters the opening of the stapler during an aligning operation in a direction substantially perpendicular to the conveying direction, a problem might be caused as the sheet is caught in the space between the staple portion 1401 and the intermediate stacking portion 1402 or in the space between the stapling portion main body 1401a and the staple cartridge portion 1401b, as shown in FIG. 16.
To counter this problem, the following measures have been taken in conventional apparatuses. As shown in FIG. 17, a guide plate 1403 is provided on the bottom face of the opening of the stapling portion 1401. Alternatively, a guide member 1404 that extends upward from the stacking face as it nears to the opening of the stapling portion 1401 may be provided on an intermediate stacking face in the vicinity of the opening of the stapling portion 1401, as shown in FIG. 18.
In those structures, sheets can be effectively prevented from being caught in the space or the step between the portions. However, since the space between the portions remains unfilled, the above measures are not effective for sheets with large curls.